A 3-year-old boy from rural Maharashtra presents with a 2-week history of progressive abdominal distension, constipation, and failure to pass meconium beyond the neonatal period. Physical examination reveals a distended abdomen with visible peristaltic waves. Abdominal radiograph shows dilated loops of small bowel with air-fluid levels. Contrast enema reveals a transition zone in the rectosigmoid region with a narrow distal segment. What is the most likely diagnosis?

Explanation

## Clinical Presentation and Radiological Findings **Key Point:** Hirschsprung disease (HD) is characterized by the absence of ganglion cells (aganglionosis) in the distal colon, most commonly affecting the rectosigmoid region. ### Pathophysiology The disease results from failure of neural crest cell migration during weeks 5–12 of gestation, leading to an aganglionic segment that cannot relax, creating a functional obstruction. ### Radiological Features | Feature | Hirschsprung Disease | Meconium Ileus | |---------|----------------------|----------------| | **Contrast enema appearance** | Transition zone with narrow distal segment | Microcolon with inspissated meconium | | **Proximal bowel** | Dilated (proximal to transition) | Dilated small bowel loops | | **Location** | Rectosigmoid (80%) | Terminal ileum | | **Associated findings** | None specific | Meconium peritonitis, calcifications | | **Age of presentation** | Beyond neonatal period (classic) | Neonatal period (cystic fibrosis) | **High-Yield:** The **transition zone** on contrast enema is the hallmark radiological finding — the abrupt change from a narrow aganglionic segment to dilated proximal bowel. ### Clinical Pearl While meconium ileus typically presents in the neonatal period with cystic fibrosis, Hirschsprung disease classically presents in infants and young children (beyond the first week) with progressive enterocolitis symptoms. The 2-week history and age (3 years) strongly favor HD. ### Diagnostic Confirmation - **Contrast enema:** Transition zone (most sensitive and specific) - **Rectal biopsy:** Gold standard — absence of ganglion cells and hypertrophied nerve trunks - **Anorectal manometry:** Absent rectoanal inhibitory reflex (RAIR) **Mnemonic:** **HD-ZONE** = Hirschsprung Disease — Zero ganglion cells, Obstruction at transition, Narrow distal, Enema shows transition